1. Field of the Invention
The present invention relates generally to the field of data storage and retrieval, and more particularly to the use of reference value comparisons and digital signal processing techniques to detect and correct data storage errors.
2. Brief Description of the Prior Art
Commonly known digital storage and retrieval devices for magnetic media operate based on two state changes in the analog magnetic flux continuum. That is, digital data consisting of a series of ones and zeros are represented on the magnetic media by magnetic flux transitions. Storage of digital data is accomplished by modulating the local field of a so-called "write head" as a function of data changes from one to zero or zero to one, and positioning media in appropriate proximity to the write head such that the field changes are recorded on the media. Retrieval of the digital data involves detecting the flux changes stored on media and translating those changes into the appropriate series of ones and zeros. The relatively high noise inherent in pure analog storage and retrieval techniques (i.e., storing and retrieving data in the form of local relative values as opposed to mere flux transitions) have heretofore proved an insurmountable barrier to their effective use.
While it is true that digital methods involve certain amounts of noise and various sources of error, many digital error correction schemes exist. A great number of these schemes utilize the fact that digital data can only validly assume two values, typically zero and one. Analog outputs are, therefore, not amenable to such error correction schemes. Thus, pure analog systems (i.e., those with analog outputs) are generally limited to applications where relatively high error rates are acceptable. Use of a digital-to-analog (d/a) converter to store data, and an analog-to-digital (a/d) converter to retrieve data has been utilized. Employment of digital data error correction techniques may then be made at certain stages of storage and retrieval. However, such arrangements result in severe noise effects from the various circuitry, conversion processes and media utilized as well documented in the prior art. This noise, especially in its analog form, cannot be adequately corrected for or filtered. Certain other hybrid approaches have been utilized in the prior art employing various correction techniques, but none have fully accounted for the varieties of errors implicit in each.
The type of errors resulting from prior art arrangements can be categorized as one of two types: (a) permanent, or (b) transient. Permanent errors are typically due to some physical characteristic of an element of the system, and are called such since they are consistent within a single system. For example, the media may have imperfections due to manufacturing processes, age, etc. Also, the frequency of signal storage may vary due to varying geometries such as that encountered when moving from one track to the next on a disk. Transient errors can be caused by the transport mechanism, such as rotational distortion resulting in wow and flutter. The circuitry itself can also be the source of transient errors, such as magnetic head charged storage, system instabilities, etc. External effects such as heat, vibration, radiation or unstable power supply can also cause transient noise. These effects result in random bit errors or a burst of bit errors and are called transient errors since they may vary within a single system (e.g., one system may introduce a bit error in one physical location on one medium yet not produce that error on a second medium).
Many error correction techniques exist, varying in complexity from a simple least-square-errors fit to a complex Reed/Solomon code. Physical differences such as varying track sizes are typically handled by precompensating the signals to accommodate for the varying geometries. Wow and flutter are typically reduced by modulating a high frequency carrier signal or introducing a reference signal and wholesale compensating the stored data as a function of detected error in storage and retrieval of the reference signal.
Prior to the present invention, there has been no method presented which takes advantage of the information imbedded in the many levels of the flux continuum, while simultaneously obtaining high bandwidth with relatively low error rates. That is, no prior art method or apparatus has been heretofore disclosed which is capable of utilizing essentially an analog spectrum with digital data accuracy.